1. Technical Field
The invention concerns a piston ring, with a main body, which exhibits a contact surface provided with a recess an upper and a lower side surface, and an inner circumferential surface.
2. Related Art
In EP-A 1 482 075, a piston ring coating is described which is made of tungsten disulfide.
The formation of the lower contact edge of a piston ring is of great importance for oil management in a combustion engine. It is a challenge in manufacturing the piston ring to produce as small an edge as possible, in order to obtain as good a stripping effect as possible during engine operation. Such small and sharp edges enable an uncoated ring to be produced relatively simply. Although today, coated rings are fundamentally necessary in the first groove of combustion engines. There is a conflict of goals here between the desired small operating edge and a blow-by- and crack-free coated edge.
One known way to manufacture relatively sharper contact edges for coated rings is the use of fully or one-sided recessed rings. A disadvantage of these rings is that a certain portion of the contact surface is not principally required to be provided with a protective coating, and in this area, in which the basic material then comes into contact with the cylinder, there often occurs so-called firing trace build-up.
The invention is based on the problem of developing a piston ring that by optimizing oil management by using a contact surface provided with at least one recess, the disadvantages mentioned are no longer present.
This problem is solved by means of a piston ring with a main body, which exhibits a contact surface provided with a recess an upper and a lower side surface, and an inner circumferential surface, whereby at least the recess is provided with a wear-protection layer, and a PVD coating layer is applied at least partially to the contact surface, to the effect that the main body in its finished state exhibits the PVD coating layer merely on the edge region side, that is, outside the recess.
Physical-vapor-deposition (PVD) coating is a process in which the coating of the contact surface of the piston ring occurs by means of deposition from the vapor phase. The coating material being deposited exists, at the same time, as a stream of ionized particles.
By means of the invention subject matter, it is now possible, by attaining a desired small-sized operating edge, to produce a blow-by- and crack-free contact surface coating in the edge area.
It is a special advantage that one-sided or completely recessed piston rings can be provided made of cast or steel materials with a galvanically or thermally applied wear-protection layer.
According to the invention, in the finished state of the piston ring, the respective edge area, that is, outside the recess is provided with the PVD coating layer. As a result of achieving this, it may be that either just the edge region is provided with the PVD coating layer, or else the whole contact surface including the recess be coated with a PVD coating layer and the PVD coating layer is subsequently removed in the area of the recess, by grinding, for example.
As a further idea according to the invention, the PVD layer is formed on the basis of nitrides of elements in groups IV B to VI B of the periodic table. Optionally, the following elements can be added, alone or in combination: Al, Si, C, O.
Especial advantageous are so-called PVD-DLC (diamond-like carbon) coatings, which are applied to the contact surface of piston rings. They form especially wear-resistant surfaces with a low coefficient of friction and are also resistant to higher temperatures. DLC layers in addition exhibit good adhesion to the main body and a high rupture strength.